Various methods have been proposed for removing and recovering acid gases, such as CO2 and H2S, contained in exhaust gases from fossil fuel combustion. In one of such methods, an amine-based absorbing solution such as an alkanolamine aqueous solution is brought into contact with combustion exhaust gas to absorb CO2 from the combustion exhaust gas. This amine-based absorbing solution having absorbed CO2 is heated to release CO2, thereby recovering CO2 and regenerating the amine-based absorbing solution for cyclic use.
In such a CO2 recovery system, it is necessary to determine optimal operation conditions from the viewpoints of ensuring the required gas absorption performance and reducing the necessary energy. It is therefore important to measure and know the amine concentration in the absorbing solution and the CO2 concentration in the absorbing solution during operation of the system. Patent Document 1 states that since the CO2 concentration in the amine-based absorbing solution correlates with the ultrasonic propagation velocity and electric conductivity of the amine-based absorbing solution, the ultrasonic propagation velocity or electric conductivity is measured, and the operation conditions are controlled by using the CO2 concentration in the amine-based absorbing solution calculated from the measured value.
Patent Document 2, which does not relate to the aforementioned CO2 recovery system, describes a technique capable of determining the concentration of each of plural (n) solutes dissolved in a solvent by measuring the ultrasonic propagation velocity in the solvent and by detecting n−1 types of specific properties for each solute. Patent Document 3 describes calculation of the concentration of each component of a three-component mixture composed of sulfuric acid, hydrogen fluoride, and water by measuring the ultrasonic propagation velocity and one of the electric conductivity and viscosity in the mixture.